Fused salt batteries of the type discussed hereinabove employ binary salt electrolytes such as lithium chloride-potassium chloride, or ternary salt electrolytes such as calcium chloride-lithium chloride-potassium chloride. The preferred anode is an aluminum lithium alloy; the cathode is constructed from activated carbon in the form of a plate; and, as a metallic current collector from the cathode, tungsten is employed. Such a battery is fully described in U.S. Pat. No. 3,645,792, commonly owned by the Assignee of record herein. The seal between the tungsten rod and the battery container or case comprises a metallic sleeve and an inert refractory material.
The seal, per se, is not the novel aspect of the foregoing patent, it is merely disclosed as a means of passing the tungsten rod through the cell lid, the electrolyte and into the attachment adaptor which is in turn connected to the cathode. The patent refers to an earlier U.S. Pat. No. 3,472,701, for a seal that can be employed. The latter patent, also commonly owned by the Assignee of record herein, comprises a metal element in the form of a coupling, and a ceramic insulating material. The metal coupling is provided around the tungsten current carrier and engages upper and lower followers which compress a sealant material such as boron nitride. Tightening the nuts of the coupling drives the followers together, compressing the boron nitride therebetween as well as around the tungsten rod, and forms a seal.
While both patents disclose seals, neither embodiment has successfully prevented passage of materials in and out of the cell. The purpose of a seal is to prevent the electrolyte and the cell gas atmosphere from leaking out. Also, ingress of oxygen will degrade materials within the cell at both positive and negative potentials through a cyclic regeneration system which results in oxygen being continuously available at the positive potential to combine with the electrode and current carrier.
Lithium, at high chemical activity dissolved in the electrolyte, cannot be permitted to contact glass in seals inasmuch as it reduces the oxide therein to form materials that are electrically conductive and will short the seal across the glass. Lithium in contact with brazing metals forms alloys therewith, rendering the metals brittle or low melting, leading to failure at a brazed joint at the same electrical potential (negative) as the cell case. Therefore, it is necessary that a seal for fused salt batteries not permit the passage of liquids or gasses in or out of the cell inasmuch as either can lead to material degradation within the cell. Also, the components employed to form the seal and protect the tungsten rod must be impervious to attack from the electrolyte, chlorine and dissolved lithium.